Digital telecommunication between various localized networks is costly because of differences in digital signal hierarchies, encoding techniques and multiplexing strategies. For example there are many different signals in the well-known Digital Multiplex Hierarchy (DMH). At the bottom of the DMH hierarchy is a basic voice channel signal having a rate of 64 kilobits per second (kbps) and known as “DS-0”. A “DS-1” signal, used in leased lines commonly referred to as “T1” lines, consists of 24 DS-0 signals and one framing bit per frame and has an overall rate of 1.544 megabits per second (Mbps). An “E1” signal consists of 30 DS-0 signals and 2 channels for framing and signaling, providing an overall rate of 2.048 Mbps. A T3 signal consists of 28 DS1 signals plus framing information, providing an overall rate of 44.736 Mbps. Communication between such diverse networks requires complicated multiplexing/demultiplexing and coding/decoding processes to convert a signal from one format to another format and to cross-connect signals between lines.
The SONET (Synchronous Optical Network) protocol was developed to standardize rates and formats. SONET is specified in American National Standards Institute (ANSI) Recommendation T1.105. SONET allows data streams of different formats, such as T1, T3, and E1, to be combined onto a single high-speed fiber optic synchronous data stream. G.707 is a standard from the International Telecommunications Union (ITU) that implements a synchronous network protocol similar to SONET, referred to as a synchronous digital hierarchy (SDH). As used herein, the term synchronous network encompasses SONET, SDH, and any similar hierarchical network.
In the example of SONET, a synchronous transport signal (STS) is the basic building block of SONET optical interfaces. The STS consists of two parts, the STS payload (carrying data) and the STS overhead (carrying signaling and protocol information). A signal is converted to STS and travels through various SONET networks in the STS format until it reaches line terminating equipment, which converts it back to the user's format.
An STS level one (STS-1) is the basic signal rate of SONET, with a rate of 51.84 Mbps. STS-1 uses a frame length of 125 μsec or conversely a frame rate of 8000 frames per second using a corresponding optical carrier level-1 (OC-1) optical signal. Higher data rates are transported using SONET by multiplexing N lower level signals together. To this end, SONET defines optical and electrical signals designated as OC-N (optical carrier level-N) and STS-N (synchronous transport signal level-N), where OC-N and STS-N have the same data rate for a given value of N. Accordingly, just as STS-1 and OC-1 share a common data rate of 51.84 Mb/s, OC-3/STS-3 both have a data rate of 155.52 Mb/s. The STS-3 frame combines three STS-1 frames in a time-division-multiplex fashion wherein the first time slot contains the first byte of the first STS-1 frame, the second time slot contains the first byte of the second STS-1 frame, the fourth time slot contains the second byte of the first STS-1 frame, and so on. An STS-48 frame is associated with an OC-48 signal at 2.5 Gigabits per second (Gbps) and includes data interleaved from forty-eight STS-1 frames as described above.
Various DMH signals may be included in the synchronous payload envelope (SPE), and the SONET standard is sufficiently flexible to allow new data rates to be supported, as services require them. In a common implementation, DS-1 signals are mapped into virtual tributaries (VTs), which are in turn multiplexed into an STS-1 SPE, and are then multiplexed further into an OC-N signal such as OC-48. The payload of a particular SPE may be associated with one of four different sizes of virtual tributaries (VTs). The VTs are VT1.5 having a data rate of 1.728 Mb/s, VT2 at 2.304 Mb/s, VT3 at 3.456 Mb/s, and, VT6 at 6.912 Mb/s. A superframe consists of four STS-1 frames, and is used to transmit a VT. The alignment of a VT within the bytes of the payload allocated for that VT is indicated by a pointer contained within two VT pointer bytes, which contain a pointer offset similar to the STS-1 pointer described below.
When cross-connect switching either STS-level or VT-level signals within SONET based communications systems, known cross-connect switches use a technique known as time-space-time switching. These cross-connect switches receive multiple system inputs that are not frame aligned, and are possibly asynchronous to each other. The cross-connect switches synchronize the inputs and column-align them in time by a technique known as pointer processing (the first time component). The pointer processed inputs are then brought together by storing this column aligned data in a large central memory (the space component). Finally, the system output selects the correct data column from the memory to create the output framed data signal (the final time component).
An important feature of these switching systems is their ability to be made non-blocking. A non-blocking system is capable of switching any input VT signal to any output VT time slot. To be considered non-blocking there must not be any combination of input VT signal to output VT time slot that cannot be connected. To create non-blocking switching systems using conventional time-space-time switching there must be a dedicated memory read port for each output because every VT output may request the same VT input simultaneously. However there are physical limitations to the number of read ports available on time slot interchange (TSI) memory devices. This physical limitation forces system architectures to implement structures such as systolic memory arrays (described below in FIG. 5) as the number of system inputs increases. However the number of column storage memory devices required to implement these systolic arrays increases exponentially as the number of inputs and/or outputs increases. Thus some elements of this type of system, including the input framers, pointer processors and output frame formatters, grow linearly as the number of inputs increases. However the column storage memory grows exponentially with the number of inputs making systems with large numbers of inputs prohibitive. It would be desirable, then, to eliminate this exponential growth relationship in known SONET cross-connect switches as the number of inputs increases. Such a cross-connect switch and related methods are provided by the present invention, whose features and advantages will be more clearly understood from the following detailed description taken in conjunction with accompanying drawings.